An optical cable is constructed with one or more optical fibers contained within a single, flexible outer sheath. Each fiber includes a light transmissive core encircled concentrically by cladding, a material with an optical index of refraction less than that of the core. An optical signal propagating within the core is retained in the core by refraction at the boundary of the core against the cladding. A bend in an optical fiber establishes a change in the angle of incidence of the optical signal with the boundary, allowing some of the signal to escape as a loss in signal rather than to refract and remain internally of the core of the fiber. To reduce losses of optical signals due to bending of the fiber, the cable is constructed to resist bending of the fiber beyond a minimum radius of curvature. For example, the cable is constructed such that each fiber is engaged by and is concentrically encircled by a flexible buffer. In turn, the buffer is encircled by an outer flexible sheath. The buffer is a solid polymeric material having an outer diameter many times larger than the diameter of the encircled fiber. When bending forces are applied the the cable, the buffer and the outer sheath are flexible and tend to bend in an arc, thereby resisting a tendency for the cable to bend acutely beyond a minimum radius of curvature.
An optical cable constructed with a single buffer covered optical fiber is known as a single fiber cable. An optical cable constructed with two or more buffer covered optical fibers is known as a multiple fiber cable. Each fiber is slender and is easily fractured when subjected to tensile force. To resist a tensile force that might fracture each fiber of an optical cable, the cable is constructed with elongated strength members contained in the sheath of the cable and extending axially of each fiber. The strength members project between the sheath and the buffer covered fibers. For example, the strength members are elongated slender fibers of a high tensile strength material, such as Kevlar, a trademark of E. I. du Pont de Nemours & Company, Wilmington, Del.
It is often desired to provide a breakout in a multiple fiber cable. A breakout is defined as a transition along a multiple fiber cable wherein one or more optical fibers leave the multiple fiber cable and become single fiber cables that are capable of being routed to individual locations. Prior to the invention a breakout was accomplished according to the following procedure. First, the outer sheath was opened at a location where the breakout was desired, thereby to expose one or more buffer covered, unsheathed fibers together with some strands of the strength members. A single fiber cable was constructed around each unsheathed fiber and some strands of the strength members. The strength members were separated into groups and were apportioned among the unsheathed fibers, whereby a group of some of the strength members accompanied each unsheathed fiber. Each unsheathed fiber and a group of some of the strength members were inserted into open ends of flexible tubing and were assembled in and along the tubing, thereby to simulate the construction of a single fiber cable. Adhesive backed tape was wound around portions of the unsheathed fibers extending between the open ends of the tubing and the opened outer sheath of the multiple fiber cable.
The prior transition has been unsatisfactory. For example, inserting the unsheathed fibers and the strength members along tubing has been a difficult and time consuming task. Accordingly, there has been a need for a transition that is constructed with relative ease. In addition, the unsheathed fibers of the prior transition have been susceptible to excessive bending where the unsheathed fibers extend between the multiple fiber cable and open ends of the tubing. Accordingly, there has been a need for a transition to include strain relief means for preventing excessive bending of unsheathed fibers extending from the multiple fiber cable to the single fiber cables.
In the prior transition, adhesive tape was applied to cover the unsheathed fibers and open ends of the tubing. The adhesive tape formed a mass of unpredictable size and shape. The fibers were unconstrained by the tape and projected from the transition in unpredictable directions. Thereby, the size and shape of the prior transition has been unpredictable. There has been a need for a transition along a multiple fiber cable, which transition has a predictable construction, size and shape.